As the standard of living improves and the demand for a more comfortable life spreads with time, the awareness of environmental issues is increasing each day. Especially, artificially emitted sources currently represent an important proportion of air pollutants due to industrialization and the advancement of industries in contrast to the past when naturally emitted sources accounted for most of the air pollutants, and the generation of nitrogen oxides (NOx) during combustion is inevitable at sites using fossil fuels, such as chemical plants or power plants, and the amount of NOx emission from mobile pollution sources, such as automobiles and vessels, accounts for the highest percentage of air pollutants. NOx exists mostly in the form of NO, but in air, NO not only induces the greenhouse effect by oxidizing easily into NOx, which is harmful to humans, but also acts as a causative element of photochemical smog that is 310 times more effective than CO2 by combining with oxygen in the presence of ultraviolet rays. In addition, since NOx is a cause of acid rain besides sulfur oxides, causes harm such as a chronic respiratory disease by inducing toxicity in the respiratory tract when inhaled into the body of animals including humans, it should be removed before being released into the air.
In the case of thermoelectric power plants in South Korea, coal, petroleum, and LNG account for about 60% of the energy sources required for the generation of electricity, indicating that those thermoelectric power plants are highly dependent on fossil fuels, and such a proportion is still on the rise. Such thermoelectric power plants currently release NOx at a level of 100 to 1,300 ppm, and, due to the reinforcement of environmental laws, the emitted sources are regulated to be in the range of 70 to 200 ppm. In addition, with the regulation of NOx emission in accordance with the International Maritime Organization (IMO) and the effectuation of Euro VI, environmental laws are currently being applied to mobile pollution sources such as automobiles and vessels.
Such methods of reducing NOx emissions include pretreatment techniques that suppress the generation of NOx by modifying combustion conditions and post-processing techniques that handle NOx that has been generated. The post-processing techniques may be classified into wet processes and dry processes, depending on the use of an aqueous solution. However, there is a limit to the pretreatment techniques that improve combustion conditions, and the pretreatment techniques are far less effective for reducing NOx emissions compared to the post-processing techniques.
Among the above, the selective catalytic reduction (SCR) process, which is a dry process that is excellent in terms of removal efficiency and economic feasibility, is being commercially widely used.
The SCR process utilizes a reducing agent such as NH3, urea, and a hydrocarbon for NOx removal, and such reducing agents are used with a catalyst phase to reduce NOx into a gas such as N2 or H2O, which is not harmful to the human body. When ammonia is used as the reducing agent, an offensive odor and toxicity result. Also, ammonium sulfate produced as SO3, which is obtained through the oxidation of SO2 among the components in the exhaust, reacts with unreacted ammonia in the exhaust is a main cause of the corrosion of equipment, thus reducing the service life of the equipment. Therefore, in preparing a catalyst, the SOx conversion ratio should be taken into account.
Catalysts based on V2O5—WO3—TiO2, which were recently commercialized and are being most widely used, exhibit high catalytic activity and strong adaptability to the environment, and thus are the catalysts most widely used for handling the NOx present in exhausts from power plants and incinerators. Among the components, the use of V2O5, which is a catalytically active component, is generally limited to an amount of about 0.3 to 1.5 wt % vanadium (V), because, although V2O5 functions to increase catalyst activity by catalyzing the conversion of NOx into N2 through a redox cycle, the addition of an excessive amount of V2O5 results in the oxidation of NH3 into N2O within the high-temperature region.
In addition, higher vanadium contents lead to higher values of the SOx conversion ratio, and, when released into the environment as a heavy metal, vanadium gives a significant impact on human health or on the environment. Also, it has been reported that the price of vanadium in 2015 is expected to reach 75 dollars per kilogram due to the recent consistent increase in demand and price. Therefore, reducing vanadium consumption is required.